322 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



the epidermis. Presently this begins to bulge upwards like a dome 

 into the epidermis. The epidermal cells immediately bounding this 

 little dermal elevation take on a columnar shape : they constitute 

 the enamel epithelium (Fig. 160, e). The (dermal) cells on the 

 surface of the dome or papilla, immediately underlying the enamel 

 epithelium, also become distinct and form a definite layer of odonto- 

 blasts. 



The hard substance of the scale makes its appearance, as a cone 

 of dentine fitting over the surface of the dermal papilla arid in turn 

 ensheatbed by the enamel epithelium. The dentine cone, which 

 usually becomes directed tail wards, gradually thickens, encroaching 

 upon the dermal papilla or pulp which it surrounds. It lies 



E 



Fig. 160. — Longitudinal vertical section through the skin of an embryonic Shark to show 

 a developing placoid scale. (Prom Balfour's Embryology : figure by Gegenbaur after 

 Hertwig, 1874.) 



E, epidermis ; e, enamel epithelium ; o, enamel ; p, dermal papilla. 



immediately outside the odontoblasts and as it increases in thickness 

 the outer portion of some of the odontoblasts persists as a fine 

 thread of cytoplasm extending out through the substance of the 

 dentine, so that when dried the dentine is seen to be traversed by 

 innumerable fine slightly diverging canals each of which contained a 

 protoplasmic thread. 



The hard material of the dentine is commonly regarded as 

 calcified matrix but there is evidence which points rather to its 

 being formed of modified cell cytoplasm. This point will be returned 

 to in connexion with the development of the teeth. 



Towards the surface of the cone the calcified substance changes 

 its character. It becomes extremely hard (consisting of very dense 

 calcium carbonate), transparent and highly refracting, and the 

 terminal branches of the tubules within it are reduced to an extreme 

 degree of fineness. This outer layer is commonly known as enamel. 

 It may be comparatively thick, or on the other hand it may be 

 extremely thin as for example in the scales of Acanthias except the 



